Problems:
  - some specs fails from time to time due to rounding errors when calculating the number of visible tiles: multiple runs will solve the problem
  - I had to duplicate some code in board_spec.rb to keep the private methods private
  - I didn't find a way to remove the caller from pry output when calling binding.pry


EXAMPLE USAGE:

$~ exe/minesweeper --width 10 --height 24 --difficulty easy
[1] pry(#<Minesweeper::Game>)> display
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXX3XXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXX3XXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXXX
XXXXXXXXX0
XXXXXXXXXX
XXXXXXXXXX
=> nil
[2] pry(#<Minesweeper::Game>)> attempt(100,100)
StandardError: Invalid coordinates
from /home/ngw/Projects/minesweeper/lib/minesweeper/game.rb:52:in `validate_coords'
[3] pry(#<Minesweeper::Game>)> attempt(2,3)
=> true
[4] pry(#<Minesweeper::Game>)> attempt(2,4)
=> true
[5] pry(#<Minesweeper::Game>)> attempt(2,5)
=> true
[6] pry(#<Minesweeper::Game>)> attempt(2,6)
=> true
[7] pry(#<Minesweeper::Game>)> attempt(2,7)
=> true
[8] pry(#<Minesweeper::Game>)> attempt(2,8)
BOOM!!!
=> nil
[9] pry(#<Minesweeper::Game>)> display
22101*3**2
**1023*6*3
34322*4**2
*3**32*431
13**212*10
0123211110
1111*21111
2*1123*11*
*3313*3222
2*2*3*21*2
112121124*
00000112**
000001*332
2221123*21
**3*33*32*
223*3**211
1222333211
*3*32*23*2
35*4*4*3*2
**4*3*2211
**42311000
344*222100
1**22**111
122112211*
=> nil